Many present-day printed circuit boards are double-side-plated, thru-hole circuit boards, typically formed from fiberglass reinforced resin panels having conductive circuit patterns etched on both sides of the card. Holes in the circuit card are a mechanical mount for electronic components and a "feed thru" for electrical conduction in etched circuits on the "front" side of the board to etched circuits on the "back" side. Electrical conduction occurs between the "front" and "back" etched patterns of the circuit board, because the inside circumference of the holes is plated with a conductive metal such as copper.
In the fabrication of such a circuit board, card holes are drilled and burrs are removed. Then, the panels and thru-holes are processed by chemical application of a thin layer of "electroless" copper. This deposition provides an initial conduction path in the hole walls, which with the board itself, will be later electroplated.
It is impractical to provide a thickness of copper on the hole walls by the chemical process sufficient for a mechanically and electrically stable "front" to "back" connection. The thin, electroless copper conductive layer is next plated by electrodeposition to build up a layer of copper in the hole walls. Before electroplating, circuit panels are typically coated with a plating resist compound which covers the circuit panel, except for the etched circuit pattern, and the holes. Photographic imaging of a circuit pattern on a light sensitive resist material, which after development exposes the circuit pattern to be subsequently plated and etched, is one means by which this is done.
When the circuit panels are electroplated, the area plated consists of (1) the circuit pattern, (2) the walls of the holes and (3) usually a "robber area" around the perimeter of the circuit pattern. These areas are plated with copper by electrodeposition, and a second electroplating operation deposits a solder coating on the circuit pattern and hole walls. The solder is also an etch resist in latter stages of fabrication. Other materials such as gold and silver may also be plated on the circuit board.
In electroplating, the thickness of the plated material deposited is proportional to the product of (1) electric current per unit area and (2) time. Given a predetermined plating current, the thickness of the plating material deposited per unit time will be inversely proportional to the surface area plated.
It is important that an exact quantity and quality of deposit be obtained on the surface of the circuit pattern and hole walls, because (1) too little a deposit results in poor mechanical and electrical characteristics for the plated thru-holes; (2) overplating may close down the hole diameter, resulting in an inability to insert components into the holes; and (3) plating at too high or too low a current density will result in a dull or burned deposit. An occurrence of any of the foregoing may result in a quality control reject. Similarly, expensive plating materials such as gold or silver may be used, and accurate control of plating parameters, as well as the avoidance of reject boards, is a critical factor in manufacturing expense.
Thus, there is a need accurately to compute the entire area of an electronic circuit board to be plated so that proportional parameters of plating current and time can be accurately determined for the plating operation for a given circuit board. Because circuit trace geometries including hole wall areas are complex, even in a simple circuit pattern, accurate mathematical computation of the surface are to be plated is difficult.
Optical techniques are used in the prior art. In one commercial device, a photographic film having the image of the circuit traces is placed between a light source and photosensors. The amount of light transmitted through the film is measured and interpreted to estimate the area to be plated. This process is inaccurate because of the inability to provide a uniform light source and optical sensing system over the entire area of the board. Thus, the light measured for a given photo image is a function of its position with respect to the optic system; and refraction, reflection and absorption characteristics may distort the light transmission. Therefore, a same surface area may yield different measured values across the area of the optical system. Optical estimation of the area of a surface trace pattern may also neglect the area gained by the hole walls, and area lost by the "faces" of the cylinders created by the holes, which in many cases substantially contributes to the overall area to be plated.
Other means to determine the area of a circuit to be plated include educated guessing, repetitive tests of different current values and current measurements in the electroplating bath dependent on a probe measurement of a "test area" of known surface area in the plating bath with a sample part to be plated. With respect to the latter, the current density of current passing through the sample panels and probe is expected to yield the plating area per part. Because of field effects, the measured value of current for a probe device depends on probe geometry as well as its total surface area and position in the electroplating bath. Consequently, computed areas vary with probe geometry and position. Additionally, such an empirical method may require repeated tests, resulting in production delay and wasted reject panels.